Rather than remove them after they have joined, why not stop them joining in the first place?
Have a read of the following for help and ideas
Please visit Preventing Spam in phpBB3
for various options for fighting spam in phpBB.
How to configure Q&A
- Administration Control Panel (ACP) > Spambot countermeasures> Installed plugins > click the dropdown box and select Q&A (it will be greyed out)
- Click the configure button then click add
- Add your question and answer (you need a Q&A for each language you have installed) click submit
- Click "back to previous page" then click "back"
- Select Q&A from the dropdown box (again)
- Click submit at the bottom of the page.
See How to clean up a board hit by spam
for some suggestions.